The present invention relates to a variable valve operating system of an internal combustion engine enabling working angle and phase to be varied, and in particular being capable of changing both working angle and phase of engine valves depending on engine operating conditions, and specifically to a variable valve operating system employing a variable working angle control mechanism and a variable phase control mechanism both used for an intake valve.
The open periods of intake and exhaust valves are usually overlapped in the last stages of exhaust stroke. A valve overlap of predetermined degrees of crankshaft rotation is effective to enhance an air intake and exhaust efficiency. However, during the valve overlap at low load operation in which a negative pressure in the intake pipe develops greatly with a throttle valve kept at a relatively small value, there is a possibility of back-flow of a large amount of exhaust gases. This deteriorates combustion stability. Undesirably increased valve overlap causes rough idling. Therefore, it is desirable to variably control the valve overlap depending on engine operating conditions. Such a variable valve timing control system capable of variably controlling the valve overlap of an intake/exhaust valve has been disclosed in Japanese Patent Provisional Publication No. 59-103910 (hereinafter is referred to as JP59-103910). JP59-103910 teaches the positive use of internal exhaust gas recirculation (EGR) for the purpose of rapid activation of catalyst (improvement of catalyst temperature rise characteristic) during cold engine operation. Concretely, in order to increase the valve overlap and thus to enhance the internal EGR effect, during the cold engine operation, an intake valve open timing (IVO) and an intake valve closure timing (IVC) are both advanced, while an exhaust valve open timing (EVO) and an exhaust vale closure timing (EVC) are both fixed. This enhances a catalytic conversion efficiency during the cold engine operation. However, owing to the valve overlap increased by varying IVO and IVC, there is an increased tendency for unburned hydrocarbons adhered to the piston lands to be exhausted via the exhaust pipe to the atmosphere. To avoid this (i.e., to reduce hydrocarbon emission during cold engine operation), Japanese Patent Provisional Publication No. 7-109934 (hereinafter is referred to as JP7-109934) teaches the provision of advanced EVC as well as advanced IVO during the cold engine operation.
When using a hydraulically-operated actuator as an actuator for the variable intake-valve working angle control mechanism and/or the variable intake-valve phase control mechanism, the hydraulically-operated actuator provides several merits, for example, simplicity in construction, a great driving force, and the like. On the other hand, the hydraulically-operated actuator has demerits, e.g., a high coefficient of viscosity during cold engine starting, that is, a mechanical response delay. In particular, during the cold engine starting at low engine speeds, a discharge of working fluid discharged from a hydraulic pump serving as a working-fluid pressure source for the actuator is insufficient, and additionally the coefficient of viscosity of working fluid is high. During the cold engine starting period, if the valve overlap is positively controlled to a direction increasing the valve overlap by variably controlling the intake/exhaust valve timing, the actual valve overlap tends to be increased more excessively than need be. This lowers combustion stability. One way to improve the mechanical response delay of the actuator is to increase the discharge of working fluid from the hydraulic pump. In such a case, there are several problems, that is, an increased driving-torque loss, deteriorated fuel economy, and increased production costs. A combination of the variable working angle control for intake valve and the variable phase control for intake valve can offer a reduction in friction loss due to an effectively reduced working angle and a reduction in pumping loss due to effectively advanced IVC, thus improving fuel economy. Additionally, a valve lift can be set to a small value owing to the effectively reduced working angle. This helps atomization of fuel spray injected, thereby effectively reducing unburned hydrocarbons.
Accordingly, it is an object of the invention to provide a variable valve operating system of an internal combustion engine enabling working angle and phase to be varied, which avoids the aforementioned disadvantages.
It is another object of the invention to provide a variable valve operating system of an internal combustion engine employing a variable working angle control mechanism and a variable phase control mechanism both driven by a supply pressure of working fluid to variably control both working angle and phase of an intake valve, which is capable of ensuring further performance enhancement and effectively improving fuel economy, while avoiding excessive valve overlap and thus enhancing combustion stability even during cold engine operation in which a temperature of the working fluid is below a predetermined temperature value.
In order to accomplish the aforementioned and other objects of the present invention, a valve operating system of an internal combustion engine enabling both valve working angle and phase to be varied, the valve operating system comprises a working angle control mechanism which variably controls a working angle of an intake valve, a first actuator which drives the working angle control mechanism in response to a supply pressure of working fluid, a phase control mechanism which variably controls a phase of the working angle of the intake valve, a second actuator which drives the phase control mechanism in response to a supply pressure of working fluid, a temperature sensor which detects a temperature of the working fluid, a control section which controls the supply pressure to each of the first and second actuators, and the control section limiting a controlled variable of at least one of the working angle control mechanism and the phase control mechanism in a cold-engine state in which the temperature of the working fluid is less than a predetermined temperature value, as compared to a hot-engine state in which the temperature of the working fluid is greater than or equal to the predetermined temperature value.
According to another aspect of the invention, a valve operating system of an internal combustion engine enabling both valve working angle and phase to be varied, the valve operating system comprises a working angle control mechanism which variably controls a working angle of an intake valve, a first actuator which drives the working angle control mechanism in response to a supply pressure of working fluid, a phase control mechanism which variably controls a phase of the working angle of the intake valve, a second actuator which drives the phase control mechanism in response to a supply pressure of working fluid, a temperature sensor which detects a temperature of the working fluid, a control section which controls the supply pressure to each of the first and second actuators, and a valve open timing of the intake valve being retarded in comparison with a predetermined stability-limit valve open timing, in a cold-engine state in which the temperature of the working fluid is less than a predetermined temperature value.
According to a further aspect of the invention, in an internal combustion engine with a valve operating system enabling both valve working angle and phase to be varied, the valve operating system comprises a working angle control means for variably controlling a working angle of an intake valve, a first hydraulically-operated actuating means for driving the working angle control means in response to a supply pressure of working fluid, a phase control means for variably controlling a phase of the working angle of the intake valve, a second hydraulically-operated actuating means for driving the phase control means in response to a supply pressure of working fluid, a temperature sensing means for detecting a temperature of the working fluid, a control means for controlling the supply pressure to each of the first and second hydraulically-operated actuating means, and the control means preventing a controlled variable of at least one of the working angle control means and the phase control means from exceeding a predetermined limit only in a cold-engine state in which the temperature of the working fluid is less than a predetermined temperature value.
According to a still further aspect of the invention, a method for variably controlling both valve working angle and phase of an intake valve of an internal combustion engine with a valve operating system having both a working angle control mechanism variably controlling the working angle of the intake valve in response to a supply pressure of working fluid and a phase control mechanism variably controlling a phase of the working angle of the intake valve in response to a supply pressure of working fluid, the method comprises detecting a temperature of the working fluid, regulating the supply pressure for each of the working angle control mechanism and the phase control mechanism depending on engine operating conditions, discriminating, responsively to the temperature of the working fluid, that the engine is conditioned in one of a cold-engine state in which the temperature of the working fluid is less than a predetermined temperature value and a hot-engine state in which the temperature of the working fluid is greater than or equal to the predetermined temperature value, and preventing a controlled variable of at least one of the working angle control mechanism and the phase control mechanism from exceeding a predetermined limit only in the cold-engine state.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.